1. Field of the Invention
This invention relates to an image forming apparatus including a transfer belt that rotates between rollers juxtaposed to each other.
2. Description of the Related Art
A developer used in an image forming apparatus may be subject to characteristic change responsive to change in environment such as temperature or humidity, or characteristic change resulting from temporal change. Change in the characteristics of the developer changes a finished state of an image obtained by charging and exposure of a photoreceptor drum and development. Hence, processes including image quality adjustment and gradation correction have conventionally been performed. According to these processes, an image of a test pattern is formed on a transfer belt, the density of the resultant image is measured by an optical sensor from above the transfer belt, and an image quality is adjusted and gradation is corrected based on a result of the measurement. In this way, change in the finished state of the image is suppressed. To measure the density of the test pattern with high accuracy, an abutting member is arranged on the rear side of a measurement point of the transfer belt.
Japanese published unexamined patent application No. 2007-121952 describes an intermediate transfer belt including a rectangular support member functioning as rear abutting portion in a point of measurement by an optical sensor. The support member has a gentle flat surface, or a contact surface determined by a part of its rear surface except grooves of 0.5 mm arranged separately in the width direction of the intermediate transfer belt. This suppresses output fluctuation of the sensor due to vibration, thereby obtaining stable output.
Japanese published unexamined patent application No. 2009-14956 describes an image forming apparatus where a reading sensor is arranged on an outer side of an intermediate transfer belt and a low-friction soft member is arranged in a position in which the low-friction soft member faces the reading sensor across the intermediate transfer belt in the rotation direction of the belt. The reading sensor and the low-friction soft member are each provided in two positions of the width direction of the belt. Vibration of the intermediate transfer belt is suppressed by making the low-friction soft member apply bias toward the belt.
Japanese published unexamined patent application No 2003-248350 describes an image forming apparatus where a toner image detection sensor and a plate-like abutting member are arranged to face each other across an electrostatic conveyor belt. The abutting member is provided on the rear side of the electrostatic conveyor belt. In this way, even if belt tension is changed, density can still be detected stably.
Japanese published unexamined patent application No 2002-182494 describes an image forming apparatus where a projecting curved surface portion of a crease suppressing member formed by curving a sheet-like member is provided in the vicinity of a toner heating portion on the rotation of an intermediate transfer belt to abut on the intermediate transfer belt over the width direction of the belt. In this way, disturbance of a toner image due to a crease in the belt caused by heating is suppressed.
In the structure described in each of Japanese published unexamined patent application Nos. 2007-121952, 2009-14956, and 2003-248350, an endless transfer belt is stretched between rotation rollers at opposite marginal portions and a sensor for reading an image density in an appropriate position on the rotation path of the belt is provided externally to the belt. This requires space for the sensor outside the rotation path of the transfer belt, resulting in corresponding size increase. Additionally, the sensor is arranged in a position corresponding to the center of an abutting member or upstream from the abutting member in the rotation direction of the belt. Hence, influence caused by vibration of the belt cannot be removed sufficiently. Japanese published unexamined patent application No. 2002-182494 does not describe an abutting member for suppressing disturbance of detection of an image density by sensor.
This invention has been made in view of the aforementioned problems. This invention is intended to provide an image forming apparatus that maintains constant detection accuracy while contributing to saving of space for an optical sensor for detecting a toner density in a test patch.